Television system



1940- A. b. BLUMLEIN 2.210.523

TELEVISION SYSTEM Filed March 1, 1958 Early! INVENTOR A. D. BL UM E/Navpfg W ATTORNEY Patented Aug. 6, 1940 UNITED s'rs rr'r Fries assignorto Electric & Musicai industries Limitcd, Hayes, Middlesex, England, acompany of Great Britain Application March 1, 1938, Serial No. 193,25?In Great Britain March 4, 193? 3 Claims.

This invention relates to television systems, and is particularlyconcerned with a modification of the system described in thespecification of copending patent application No. 59,022 filed January14, 1936, which describes circuits for obtaining synchronism between theoperation of scanning at a receiver with that at a transmitter.

In the specification of co-pending patent application No. 59,022, theproblems involved in obtaining synchronism are discussed, andalternative synchronising pulse separating circuits are described, suchcircuits being for use in television systems employing frame pulses ofrelatively long duration and line pulses of relatively short duration,but of the same amplitude. In the arrangements described in thespecification, a condenser is charged through a resistance and shunt edby a valve which is positively biased, and is, therefore, conducting inthe intervals between pulses. The condenser charges during pulses and asaw-tooth voltage waveform is produced, the pulses of which are ofduration equal to that of the pulses producing them but of greateramplitude in the case of the longer frame pulses. By applying bothderived saw-teeth pulses to an amplitude selecting device the pulses oflarger amplitude operate to produce pulses which are applied to theframe deflecting apparatus.

, The specification referred to only discloses circuit arrangementswhich depend upon the components having shorter time constants betweenpulses than during pulses. Actually the time constant between the pulsesis much shorter than the gaps between them.

The object of the present invention is to provide a pulse separatingcircuit which will give correct separation for interlacing frame pulseswhile employingtime constants which are longer during the intervalsbetween pulses than during pulses.

According to the present invention in a modification of a televisionreceiver described in the specification of co-pending patent applicationNo. 59,022, the converting means for producing derived frame and linepulses from frame and line synchronising pulses includes a condensercharged from a source of potential, a resistance connected across saidcondenser and a device for controlling the charge in said condenser thetime constant of said resistance and condenser being greater than theintervals between the synchronising pulses but rendered effectivelyshort by means of a unilaterally conducting device such as a diode valvewhich serves to maintain a predetermined voltage across the condenser inthe intervals between pulses. The device for controlling the charge inthe condenser may be a thermionic valve to the input circuit of whichthe mixture of pulses to be separated is applied.

In a particular circuit arrangement embodying the invention theresistance and condenser are connected in the anode circuit of thecontrolling valve and a diode valve or the equivalent is connectedbetween a source of potential and the anode of the controlling valve insuch a manner that the diode is only rendered conducting when the chargein the condenser reaches a predetermined value. The derived frame pulsesare applied to a saw-tooth waveform generating circuit through a diodevalve the cathode of which connected to. the anode of a controllingvalve and the anode of the diode is connected to a source of positivepotential through a coil coupled with the coils of a blocking oscillatorcircuit. I

In order that the invention may be more clearly understood and readilycarried into effect, an impulse separating circuit operating inaccordance therewith, will now be described by way of example withreference to the accompanying drawing in which:

Fig. 1 is a circuit diagram, and

Figs. 2 and 3 show explanatory waveforms.

Referring to Fig. 1 of the drawing, the valve i which althoughrepresented as a triode, may be a tetrode or a pentode, is arranged tobe selfbiassing, and a resistance 2 is connected between the rid andcathode of the valve for this purpose. Picture signals and synchronisingsignals are together feed to the grid of the valve I through a condenser3, the synchronising signals being in the positive sense and of suchamplitude that while they will cause anode current to flow in the valvel the anode current will be cut off at all picture signal amplitudes. Aresistance t is connected between the anode of the valve and the sourceof high tension voltage and a condenser 5 is connected across theresistance i. The circuit connections and method of operation up to apoint marked A in the diagram are typical of known arrangements forseparating synchronising pulses.

Diode valves 6 and iare shown having the anode of one and the cathode ofthe other connected to the anode of the valve I and derive theiroperating potentials from the high tension source supplying the chargingcurrent for the condenser 5.

The operation of the circuit will be described with reference to Figs. 2and 3 of the drawing,

the former showing a typical synchronising signal waveform comprisingthree line synchronising pulses I followed by two frame pulses f. Fig. 3shows the two curves A and B drawn with reference to a line CDrepresenting maximum high tension voltage. The curve A represents thevoltage obtained at the point A in the circuit shown in Fig. 1independently of any apparatus following that point in the circuit.

quired to fire the blocking oscillator generating On the application ofa synchronising pulse to the grid of valve l, the valve is renderedconductive and the condenser 5 tends to charge negatively, as shown bythe straight falling portions of curve A, and in the intervals betweenthe synchronising pulses the potential of the anode of valve 8 tends toreturn towards the H. T. voltage as shown by the rising curved portionsof the curve A. The time constant in this instance is such that thedischarge of the condenser 5 occupies a time somewhat greater than thatof a line period.

On the application of the frame pulses to the grid of valve l, the anodepotential tends to fall further than in the case of the line pulses, andto pass a limiting value a at which point the frame scanning apparatusis operated. It will be seen that the time taken for the potential ofpoint a to be reached, depends upon the amount of charge in thecondenser 5 left over from the preceding line synchronising pulse.

If the synchronising pulses are arranged to effect interlacing, someframe pulses will occur mid-way between line pulses and others willoccur at the same time as line pulses. Due to the 3 different positionsof alternate frame pulses with respect to the line pulses, there aredifferent charges in the condenser 5 at the occurrence of alternateframe pulses so that the times after the beginning of the frame pulsesat which the po-. tential of the condenser rises to the value rethedeflecting waveform are different, resulting in one frame scan being alittle late with respect to the other and inaccurate interlacing.

The curve B represents the voltage across the condenser 5 resulting fromthe connection of the diode valves 6, l as shown. The condenser 5 isprevented by the diode 6 from discharging to the H. T. potentialrepresented by the line CD, the cathode of the diode being connected tothe positive terminal of the high tension source through a tapping point8 in a potential divider including resistances 9, ill and Ii, the point8 being decoupled by the condenser IZ. The diode 5 is thus biassed sothat it becomes conductive only when the voltage at the anode reaches acertain value as the condenser 5 tends to discharge to the high tensionpotential.

During the interval between the line synchronising pulses, the potentialof the anode of valve I therefore rests at the value indicated by thestraight horizontal portions of the curve B. On the occurrence of asynchronising pulse the potential falls as in curve A, and at thecessation of a synchronising pulse the potential rises. If the diode 6were not connected as shown, the curve would proceed along the dottedline shown. As soon as the potential of the anode reaches the criticalvalue of the cathode of the diode G, the rise of the anode potentialceases, so that its potential is maintained fixed as shown in the fullline of curve B. It will thus be seen that in eifect an artificial shorttime constant is given to the potential rise by the diode 6. On theoccurrence of a frame synchronising signal the valve remains conductingfor a longer period, and causes the potential of the point A to dropbelow the limiting voltage applied to the diode l, the anode of which isheld through the coil l3 at the potential at the tapping point l4 whichis decoupled through a condenser 15. As soon as the potential across thecondenser 5 drops below the level b, current flows through the coil l3which may be coupled for example, with the coils of a blockingoscillator controlling the frame scanning. It will be seen fromexamination of the curve B that the time at which the potential b isreached is unaffected by any remnant of the preceding framesynchronising pulses. If desired, the diode i may be biassed from alower potential such as that represented at b, so that it will notconduct until the occurrence of the second, third or even the fourthframe synchronising pulse.

-While the valve I has been shown to be of the triode type, it will beunderstood that a valve of the screen grid or pentode type may beemployed, and in such a case the line frequency pulses de; rived fromthe line frequency synchronising pulses and fed to a line frequencysaw-tooth waveform generating circuit may be taken from the screeninggrid of the valve.

I claim:

1. In a television receiver wherein line and frame synchronizingimpulses of different duration are received, comprising a discharge tubehaving a cathode, an anode and a control electrode, means for applyingthe synchronizing impulses to said control electrode, circuit meansincluding a source of potential for maintaining said anode positive withrespect to said cathode, a resistance and a condenser connected inparallel and included in the anode circuit, a second discharge tubehaving a cathode and an anode, a second resistance, means for connectingthe anodes of the discharge tubes together, means including said secondresistance for connecting the cathode of said second discharge tube tothe positive terminal of the source of potential, and resistance meansfor connecting the cathodes of said discharge tubes together whereby thecondenser may be charged on the application of synchronizing impulses inaccordance with the length of the impulses, and whereby the condensermay be discharged to a predetermined value during a time periodmaterially less than the frequency of occurrence of the impulses.

2. A television receiving circuit including a discharge tube having acathode, a control electrode and an anode, means for applying line andframe synchronizing impulses to said control electrode, the frameimpulses being of a greater duration than the line impulses, a source ofpotential, means for connecting the negative terminal of the source ofpotential to said cathode, means including a resistance and a condenserconnected in parallel for connecting the positive terminal of the sourceof potential to said anode, a second discharge tube having a cathodeandan anode, means for connecting the anode of said second discharge tubeto the anode of said first mentioned discharge tube, aresistanceconnected across said source of potential, and means forconnecting the cathode of said second discharge tube to a point alongsaid resistance whereby said condenser may be charged on the applicationof synchronizing impulses in accordance with the length of the impulses,and whereby the condenser may be discharged toa predetermined valueduring a time period materially less than the frequency of occurrence ofthe impulses. I

3. In a television receiver wherein line and frame synchronizingimpulses of difierent duration are received, comprising a discharge tubehaving a cathode, a control electrode and an anode, means for applyingthe synchronizing impulses to said control electrode, a source ofpotential, means for connecting the negative terminal of the source ofpotential to said cathode, means including a resistance for connectingthe positive terminal of said source of potential to the anode of saiddischarge tube, a condenser connected in parallel with said resistance,a second resistance connected across said source of potential, a pair ofdiodes each including a cathode and an anode, means for connecting thecathode of one of said diodes to a point along said second resistancerelatively adjacent the positive end thereof, means for connecting theanode of the other diode to a point along said second resistancerelatively adjacent the negative end thereof, and means for connectingthe other electrode of each of the diodes to the anode of said firstmentioned discharge tube, whereby the condenser may be charged on theapplication of synchronizing impulses in accordance with the length ofthe impulses, and whereby one of said diodes may be rendered conductivea predetermined and fixed length of time after the initiation of a framesynchronizing impulse.

ALAN DOWER BLUMLEIN.

